PVC is a commodity plastic which is widely used because of its excellent resistance to oxidative attack and other chemical reactions. The plastic is very weather resistant and has found extensive use as metal coatings, cable insulation, pipe, gaskets, vessel liners, and the like. The natural polymer, however, is fairly rigid and brittle and must be plasticized to make it useful commercially for these applications. This can be readily accomplished, but it is difficult to prevent loss of the plasticizer by volatization or by migration within the plastic itself, thereby changing the properties of the PVC. There has been extensive work, consequently, to develop so called permanent plasticizers which are compatible with PVC, which then retains its valuable mechanical properties over extended periods of time.
U.S. Pat. No. 2,297,194, Badum (1942) describes one of the earliest successful attempts to plasticize PVC so that it could be used as a cable insulation. In this case the plasticizer was a copolymer of butadiene and acrylonitrile. This was also reported by Emmett, Industrial Engineering Chemistry, 36, pages 730-4 (1944), who describes PVC plasticized with butadiene/acrylonitrile rubber. The blends were made by milling masterbatches of the polymers.
As early as 1971, Hammer in Macromolecules, 4, pages 69-71, reported that compatible blends of PVC and VAE copolymers can be prepared, provided the copolymer contains 60 to 75 wt. % of vinyl acetate. Hammer was reporting the discovery of a co-worker, but he himself had worked with containing 65 to 70 wt. % vinyl acetate. He stated that the blending can be carried out on a 2-roll mill or by solution blending. In actual practice, however, processing PVC with VAE copolymers containing such high amounts of vinyl acetate is quite difficult, because of the "gum stock" character of the VAE which is amorphous and has a low Tg. Commonly used PVC processing equipment will accept solids either in particulate or powder form or liquids, but cannot deal with viscous materials having low Tg. Consequently, the commercial utility of these particular VAE copolymers as plasticizers for PVC has not developed further.
Others have attempted to overcome this problem by modifying the VAE copolymer, so that it contains a higher proportion of ethylene without destroying the compatability of the polymer with the PVC. This has been done by preparing a terpolymer which incorporates into the VAE either sulfur dioxide or carbon monoxide. Hickman and Ikeda, J. Polym. Sci., Polym. Phys. Ed., 11, pages 1713-21 (1973) described studies of blends of PVC and terpolymers of ethylene, sulfur dioxide and vinyl acetate, for example, 72.7 mole % ethylene, 18.5 mole % vinyl acetate and 8.8 mole % sulfur dioxide. Compatible blends containing up to 40% of the terpolymer were found and the blends were made on a rubber mill. On the other hand, terpolymers containing only 3.2% sulfur dioxide evidenced incompatibility.
U.S. Pat. No. 3,780,140, Hammer (1973) discloses making a plasticizer for PVC by polymerizing ethylene (40-80%), carbon monoxide (3-30%) and a termonomer (5-60%) which preferably is vinyl acetate. Product blends are said to contain 5 to 95 wt. % of the terpolymer and 5 to 95 wt % of the PVC. Films and rigid or semi-rigid articles are said to be formable by varying the proportions of these polymers. The blends are formed by solution blending the polymers or blending on a roll mill, in an extruder or in a Banbury mixer.
This work was extended by Robeson and McGrath, Polym. Eng. Sci., 17, pages 300-4 (1977) who describe miscible blends of PVC and terpolymers of ethylene, ethyl acrylate and carbon monoxide. These terpolymers are said to contrast with the ethylene/ethyl acrylate copolymers which are not miscible with PVC. Similar results were reported for ethylene/vinyl acetate/carbon monoxide terpolymers and blends were made from solutions in tetrahydrofuran followed by milling to improve homogenity.
Attention has also directed to the development of other permanent plasticizers for PVC. Robeson, J. Appl. Polym. Sci., 17, pages 3607-17 (1973) describes blends of PVC and poly-e-caprolactone (PCL) that result in a true solution. It is reported, however, that the PCL can slowly crystallize out of the solution leading to a loss in flexibility for the blend. The original blends were made from solutions in tetrahydrofuran containing the polymers followed by fluxing on a roll mill.
Walsh and McCowen, Polymer, 21, pages 1335-40 (1980) describe measuring the compatibility of polyacrylates and polymethacrylates with PVC by studying the interaction of these polymers with solvents. Matzner, Wise, Robeson and McGrath, Makromol. Chem., 183, pages 2871-9 (1982) disclose miscible polymer blends of PVC and copolymers of ethylene with N,N-dimethylacrylamide, and other compatible polymer combinations. The blending techniques involved solution blending followed by fluxing on a 2-roll mill.
Polymerization of one polymer in the presence of the other has been reported for a number of combinations in efforts to permanently plasticize PVC. Over two decades ago, Golstein, U.S. Pat. No. 3,562,354 (1971) disclosed copolymerizing vinyl acetate and either glycidyl acrylate or methyacrylate in the presence of porous grains of PVC. The porous PVC grains could be made in aqueous suspension with a dispersing agent which increases the PVC porosity. This porosity is measured by the ability of the PVC to absorb plasticizer and it is said that the PVC should absorb at least 15 and preferably at least 25 grams of plasticizer per 100 grams of the PVC. The amounts of copolymer added were 0.1 to 7 wt. % glycidyl acrylate and 0.1 to 15% vinyl acetate. The composition was said to be useful for metal coatings either by fluid bed or electrostatic processes.
U.S. Pat. No. 3,764,638 Hwa et al., (1973) discloses polymerizing an acrylic monomer, for example, methylmethacrylate, in the presence of a PVC emulsion so that the acrylic polymer is formed in or on the preformed PVC particles. The product obtained can be added to other PVC as a processing aid. The acrylic polymer can comprise 10 to 50 wt % of the product and the balance is PVC.
U.S. Pat. No. 4,115,479 Daidone (1978) describes making casting resins containing 20 to 65% vinyl resin by polymerizing a monomer such as methylmethacrylate in the presence of a vinyl resin, such as PVC, of two particle sizes, one resin 10.2 to 5 microns and the other 15 to 150 microns.
U.S. Pat. No. 4,180,447 Sencar (1979) describes making PVC foams from PVC plus a monomeric softener which is later polymerized with ionizing radiation. From 10 to 50 wt %, based on the PVC, of the monomers can be used. Examples include methylmethacrylate, but there is no suggestion to use either vinyl acetate or ethylene.
Walsh and Sham, Polymer, 25, pages 1023-27 (1984) disclose polymerizing n-butylacrylate in the presence of suspended PVC beads in water. These authors state that the PVC has been found to be miscible with a wide range of polymers containing electron donor groups, such as esters, and further that the polymer blends can be reswollen and additional n-butylacrylate polymerized to produce homogenous blends containing more than 10% n-butylacrylate. These systems, however, are not thermodynamically stable at concentrations where plasticization can be achieved. Even if miscible blends are capable of being prepared from solution, irreversible phase separation occurs once the samples are heated to 140.degree. to 150.degree. C. and, therefore, are not practical for melt processable, permanently plasticized PVC systems.
Others have approached polymerization blending from a different direction. U.S. Pat. No. 4,155,954, Buning et al., (1979) discloses grafting vinyl chloride onto powdered ethylene-vinyl acetate copolymer in a gel phase polymerization. The product can be used to improve impact strength of the PVC. The VAE copolymers contain 0.5 to 15 wt % vinyl acetate and 85 to 99.5 wt % ethylene and about 5 to 70 wt % of the product is grafted vinyl chloride.
U.S. Pat. No. 4,323,661, Kraus et al., (1982) discloses making sinterable molding compositions by graft copolymerization of 85 to 99.5 wt % vinyl chloride in the presence of VAE copolymer, so that the graft copolymer contains 0.09 to 10.5 wt % polymerized ethylene and 0.09 to 10.5 wt % polymerized vinyl acetate. The molding composition also contains free sulfonic acid and a wetting agent. The composition has particular utility in forming battery separation plates. The vinyl chloride is polymerized in suspension in an aqueous phase containing a VAE copolymer of 30 to 70% ethylene and 70 to 30% vinyl acetate and having a molecular weight of 5,000 to 200,000.
In spite of several decades of development, problems remain in methods of incorporating into PVC a VAE copolymer which has a composition making it compatible with the PVC and, therefore, can qualify as a permanent plasticizer. VAE copolymers of this type having a high vinyl acetate content are basically adhesive-type compositions and find utility in such applications. According to Hwa et al., in the '638 patent, the polymerization of an acrylic monomer in the presence of PVC occurs so that the polymer is formed in or on the preformed PVC particles. If the vinyl acetate-ethylene copolymers formed outside the PVC (for example as separate particles) free flowing particles could not result because, when the system is coagulated for recovery, the VAE particles would act as an adhesive between the PVC particles, thus yielding a blocked system without free flowing characteristics. Even if the VAE polymerized within the PVC particles and also on the particles resulting in a coating of VAE, a coagulated product would be formed which would severely block. Another potential problem is apparent in the distribution of the vinyl acetate and ethylene during the copolymerization, since vinyl acetate has a much higher solubility in PVC than ethylene. This property could yield a polymerization of a vinyl acetate rich polymer in the PVC phase and an ethylene rich polymer in the pores of the PVC. Such a compositional split would yield a microheterogeneous system based on polymers which are actually outside the composition which is miscible with PVC.
The prior references which suggest the polymerization of vinyl chloride onto ethylene-vinyl acetate copolymers to yield graft copolymers, do not provide an acceptable answer to the problem. While graft copolymers may be desired for ethylene-vinyl acetate copolymers at higher ethylene concentrations than are permissible for the PVC miscibility, grafting is not desired for the formation of a permanent plasticizer, because excessive grafting would cause major processing problems and would yield a product which resembled a material full of gels or would appear to be lightly cross-linked. Additionally, the phase separated product would not yield properties required for a single phase permanently plasticized composition. This requirement is discussed in a recent article by L. M. Robeson, J. Vinyl Technology 12(2), 89 (1990).